A fuel cell is an energy converting device that converts chemical energy of fuel into electric energy through an electrochemical reaction. The fuel cell may be used not only to supply power for industries, homes, and vehicles but also to supply power to small-sized electric/electronic products and portable devices.
In recent years, much research has been conducted into a polymer electrolyte membrane fuel cell (PEMFC) having high electric power density as a fuel cell for vehicles.
FIG. 1 is a view showing a configuration of a unit cell of a fuel cell. A fuel cell stack may have the following configuration.
A membrane-electrode assembly (MEA) 101 as a main component is located at the innermost portion of the unit cell. The membrane-electrode assembly 101 includes a solid polymer electrolyte membrane for moving hydrogen ions, and electrode layers, such as a cathode and an anode, formed by coating a catalyst on opposite sides of the electrolyte membrane.
In addition, gas diffusion layers (GDL) 102 are stacked on outsides of the membrane-electrode assembly 101, at which the cathode and the anode are located, and bipolar plates 103, each having a flow field through which reaction gases (hydrogen as a fuel gas and oxygen or air as an oxidizing gas) are supplied and a coolant flows, are located at outsides of the gas diffusion layers 102.
In addition, gaskets 104 for sealing a fluid are interposed between the bipolar plates 103. The gaskets 104 are generally provided in a state in which the gaskets 104 are integrally formed at the membrane-electrode assembly 101 or the bipolar plates 103.
The above components constitute a unit cell. A plurality of cells 110 are stacked, the stacked cells 110 are arranged between end plates, and the end plates are coupled to each other to constitute a fuel cell stack.
Each unit cell maintains a low voltage during operation. For this reason, several tens to several hundreds of cells 110 are stacked in series to increase voltage and are then used as a power generating device, the most general shapes of which are shown in FIGS. 2 and 3.
A fuel cell stack 100 is mainly assembled and fastened in a bolt fastening fashion, shown in FIG. 2, and a fastening bar fastening fashion shown in FIG. 3.
In the bolt fastening fashion, as shown in FIG. 2, cells 110 and end plates 120 are stacked with bolts 131 each having a larger length than the stack inserted through the end plates 120. Nuts 132 are fastened to opposite ends of the bolts 131 such that the end plates 120 do not move.
The bolt fastening fashion has an advantage in that it is possible to fasten and fix the components of the stack 100 in a compressed state using a fastening force of the bolts and the nuts without using additional press equipment.
In the fastening bar fastening fashion, as shown in FIG. 3, cells 110 are stacked, end plates 120 are coupled to opposite ends of the stacked cells 110, fastening bars 133 are put on the end plates 120 in a state in which the end plates 120 are pressed using press equipment, and the fastening bars 133 are fastened to the end plates 120 using bolts 134.
The fastening bar fastening fashion has an advantage in that dead volume is minimized, and therefore the fuel cell stack is advantageous in terms of packaging when applied to a vehicle.
The end plates 120 located at the opposite ends of the stack 110 pressurize the bipolar plates while supporting the bipolar plates in a state in which the stack is fastened. Fastening of the stack is achieved using mechanisms, such as fastening bars, in a state in which uniform surface pressure is maintained over the entire area of each of the bipolar plates.
After the stack is fastened, the opposite end plates 120 are drawn toward each other to maintain surface pressure. At this time, repulsive force is generated due to elasticity of the gas diffusion layers and the gaskets. As a result, tensile force is applied to the mechanisms, such as fastening bars, thus maintaining equilibrium of static force.
Surface pressure between cells has a great effect on the total output of the fuel cell stack. Since the surface pressure in the stack is directly related to ohmic loss due to the increase of contact resistance and mass transfer resistance in the gas diffusion layers, appropriate maintenance of fastening force is a requisite condition in order to obtain good stack performance.
When the surface pressure is too low, the contact resistance between the bipolar plate/gas diffusion layer/membrane-electrode assembly is increased, resulting in current-voltage drops. When the surface pressure is too high, the gas diffusion layers are excessively compressed with the result that gas diffusion is difficult, thereby lowering stack output.
Reduction of the fastening force due to aging of the stack may cause a reduction of surface pressure on the gaskets as well as reduction of the output performance of the stack, resulting in poor airtightness of the reaction gases and the coolant.
In addition, the surface pressure may be reduced due to thermal shrinkage of the gaskets applied to the bipolar plates at a lower temperature of −30 to −20 C. When the surface pressure is excessively reduced, poor airtightness may result.
In addition, when the surface pressure of the gas diffusion layers is lowered due to a reduction of the fastening force, contact resistance between the gas diffusion layer and the bipolar plate and between the gas diffusion layer and the membrane-electrode assembly is increased. This results in an increase in ohmic loss, thereby lowering efficiency of the fuel cell.
Reduction of the fastening force and the surface pressure is caused by the hardening of the gas diffusion layers due to operation of the stack and reduction of elastic force due to deterioration of the gaskets.
When the temperature of the fuel cell stack is lowered to a lower temperature approximate to a glass transition temperature Tg of rubber, which is a material constituting the gasket, elasticity of the rubber is gradually lowered. At this time, since the thermal coefficient of expansion of the gasket is higher than that of the fastening bar, which is made of metal, the gasket shrinks much more with the result that the surface pressure of the gasket is lowered.
In order to solve the reduction in surface pressure of the gas diffusion layers and the gaskets (reduction in fastening force of the stack and poor airtightness), a method of mounting an insert to the inside of each fastening bar of the stack with reduced surface pressure, a method of further inserting a dummy cell, a method of replacing each fastening bar with a short fastening bar, and a method of mounting an elastic mechanism are used.
FIG. 4 is a view exemplarily showing a case in which an insert is mounted between each fastening bar and a corresponding end plate to repair the stack. The stack where a fastening force (surface pressure) has been lowered due to the hardening of the gas diffusion layers is further pressurized using press equipment. Then an insert 141 is mounted between each fastening bar 133 and a corresponding end plate 120 to increase the fastening force (surface pressure) to a desired level.
At this time, the length Lrepaired of the stack after repair is shorter than the original length Loriginal of the stack before repair by the length Linsert of the added insert.
In addition, FIG. 5 is a view exemplarily showing a case in which a dummy cell is further inserted between one of the end plates and a corresponding reaction cell to repair the stack. The stack of which fastening force (surface pressure) has been lowered is disassembled, a dummy cell 142 is inserted between one of the end plates 120 and a corresponding reaction cell 110, the end plates and the cells are further pressurized using press equipment, and then the end plates, the dummy cell, and the reaction cells are fastened using other fastening bars 133 each having a length suitable for increasing the fastening force (surface pressure) to a desired level.
In this case, the length Lrepaired of the stack after repair may be different from the original length Loriginal of the stack before repair due to the insertion of the dummy cell 142.
FIG. 6 is a view exemplarily showing a stack having an elastic mechanism mounted therein to prevent a reduction of fastening force and surface pressure. When the surface pressure is lowered in a stack in which an elastic mechanism, such as springs 144, is mounted, the length of the stack is automatically reduced in real time by force of the springs in order to minimize a reduction of the surface pressure. The length of the stack decreases with the increase in length of the springs.
At this time, the elastic mechanism is structurally weak in a direction perpendicular to a cell stacking direction. When the stack is mounted in an enclosure or a frame, therefore, it is necessary to use a pressure plate 143 and an opposite end plate B.
As a result, the length Lrepaired of the stack after deformation of the elastic mechanism may be different from the original length Loriginal of the stack before deformation of the elastic mechanism.
Application examples of such an elastic mechanism are disclosed in US Patent Application Publication No. 2009-0004533, No. 2005-0277012, and No. 2007-0042250.
Meanwhile, in a case in which the above methods are applied to solve a reduction of the fastening force and surface pressure, the length of the stack in the cell stacking direction is changed. Consequently, the following problems are caused when mounting the stack in the enclosure or the frame.
FIGS. 7 and 8 are views exemplarily showing conventional stack mounting structures. As shown in the figures, end plates 120a and 120b (a pressure plate for a stack having an elastic mechanism applied thereto) located at the opposite ends of the stack 100 are fastened and mounted to the enclosure or the frame 200 using brackets 151 and bolts 152 in a completely fixing fashion.
That is, the opposite ends of the stack 100 are completely fixed to the enclosure or the frame 200 in a fixed-fixed condition state.
When the cells are stacked in an X direction, therefore, translation and rotation of first and second mounting parts located at the opposite ends of the stack in X, Y, and Z directions are restricted. That is, a degree of freedom in all directions is restricted at the opposite ends of the stack. As a result, it is not possible to actively respond to the change in length of the stack by repair or by the use of the elastic mechanism.